1. Field of the Invention
The present invention relates to the cleaning of wellbores in the field of oil and gas recovery. More particularly, this invention relates to a device adapted to improve the erosion performance of components utilized in the cleaning of solid particulate matter from a well.
2. Description of the Related Art
In the oil and gas industry, wellbores often become plugged with sand, filter cake, or other hard particulate solids, which need to be removed periodically to improve oil production. Prior art methods for cleaning the wellbore and the removal of these particulate solids include pumping a fluid from the surface to the area to be cleaned. To effectively clean the solids from the wellbore, the pumped fluids must return to surface, thereby establishing circulation. Therefore, the bottom of the hole circulating pressure must be high enough to support circulation but low enough to prevent leak off into the reservoir. In addition, the fluid must suspend and transport the solids. The fluid velocity and Theological properties must support solids transport.
It is known that the bottom hole pressure of a wellbore declines as the reservoir matures, thereby complicating the wellbore cleanout. For example, if the fluid being pumped into the wellbore exits the work string (e.g., coiled tubing) at an excessive pressure, the fluid may enter the formation instead of returning to the surface with the sand particulates.
To overcome this problem, it is known to utilize gasification (e.g., by the addition of nitrogen to the fluid) to decrease the hydrostatic pressure in the wellbore. Thus, the fluid may be pumped at reduced bottom hole pressures and circulation through the wellbore may be restored to transport the particulates to the surface. However, over time, the reservoir pressure may decline to a point whereby gasification fails to result in consistent circulation of fluid to effectively remove the particulates.
Reverse circulating is another method commonly used to increase the transport velocity of the fluid, especially when employing small diameter tubing in large wellbores.
Yet another prior art method of removing the particulate solids in the wellbore where the bottomhole circulating pressure is a concern employs a jet pump, as described in U.S. Pat. No. 5,033,545 to Sudol, issued Jul. 23, 1991, incorporated by reference herein in its entirety. The jet pump is attached to a coiled tubing inside coiled tubing string (CCT). The power fluid is pumped down the inner string and returns, both the power fluids as well as the reservoir fluids, are taken up the coiled tubing coiled tubing annulus. The jet pump is designed such that reservoir fluids enter the pump at the bottom hole pressure (BHP). The jet pump then increases the pressure of the fluid pumping the fluids up the work string with the solid particulates entrained in the fluids. Thus, circulation is facilitated as the circulation no longer depends on BHP alone.
FIG. 1 shows an exemplary prior art jet pump apparatus (BHA) and method for effectively removing particulates such as sand from within a wellbore. The jet pump is particularly well suited for use with coiled tubing. The following is a simplified summary of the operation of this apparatus and method. A jet pump 5 is shown within a wellbore. The jet pump 5 is attached to the bottom of CCT (not shown) via housing 6. In operation, fluid is pumped down the inner coiled tubing (from left to right in FIG. 1). The fluid enters the BHA and ported into the lower end of jet pump 5 as shown by the arrows. As the fluid passes through nozzle 1, the velocity of the fluid increases significantly, creating a jet stream. This increased velocity creates a low pressure that is felt at the entrance 7 to the jet pump 5. The low pressure draws fluid and solid particles into the jet pump. Subsequently wellbore fluids and solids contained therein are entrained into the jet stream. The high-velocity fluid with sand particulates then enters the entrance end of the throat 100. As the fluid with the sand particulates continues to travel upward through the throat 100, the diameter of the throat increases, the velocity of the fluid decreases, and the fluid pressure increases.
This method is commonly practiced with the use of coil-in-coil tubing, as described in U.S. Pat. No. 5,638,904 by Misselbrook et al., issued Jun. 17, 1997, incorporated by reference herein in its entirety.
It has been determined that in some applications, the high-velocity impact of the sand-ladened fluids with the entrance of the throat causes excessive erosion in the high impact area 2. Other methods to remove particulate solids which utilize a nozzle, a throat, or a diffuser for entraining the sand-water slurry environment also experience excessive erosion. This erosion is generally most prominent at the nozzle, throat, or diffuser, as these are the pinch points for the flow of fluid and are associated with higher velocity streams.
Erosion of the downhole tools may be exasperated when cleaning particulates from deeper wells. Deeper wells produce additional challenges for the above-referenced procedure, as the deeper wells have increased hydrostatic pressure and increased friction pressure. Thus, the coiled tubing operation must incorporate higher pump output pressure and higher jet velocities in the nozzle and throat. For example, it is not uncommon for 8600 foot well to have 1000 p.s.i. bottom hole pressure, causing the flow velocity through the throat to be between 200 and 600 feet per second. These higher particle laden jet velocities increase the erosion rate in the throat.
Thus, there is a need for a device for improving erosion performance of devices used in the cleaning of a wellbore, such as nozzles, throats, or diffusers utilized downhole. The device should resist erosion associated with the high velocity jets of sand/water slurries generated when removing particulate solids, such as sand, from the wellbore during well intervention or workover.
It is also known to decrease the erosion of the components of downhole tools by manufacturing the components of various materials, such as ceramics like TTZ stabilized zirconia, or 6% submicron tungsten carbide. However, these prior art methods fail to provide the desired level of erosion performance and may not be economically feasible with deeper wells (and the concomitant increase jetting velocities), as excessive erosion may still result. Thus, there is a need for improving the erosion performance (i.e. decreasing the erosion) of components used in the cleaning of a wellbore when the components are exposed to high velocity sand/fluid slurries.